1. Field of the Invention
The present invention relates to a surface acoustic wave device having a filter characteristic, especially, a balance-unbalance conversion function, and a communication device including such a novel surface acoustic wave device.
2. Description of the Related Art
In recent years, techniques for reducing the sizes and weights of portable telephones have remarkably progressed. In order to achieve such reduction in size and weight, composite elements each having a plurality of functions combined with each other have been developed in addition to reduction of the number and sizes of components. Under these circumstances, recently, surface acoustic wave filters having balance-unbalance conversion functions, namely, balum functions for use in the RF stages of portable telephones have been intensively studied. The GSM system (Global System for Mobile Communications) and other such telecommunications systems have been used.
When balanced lines such as a two parallel line feeder are connected directly to unbalanced lines such as a coaxial cable, undesirably, unbalanced currents flow, and the feeder itself acts as an antenna. Thus, the balun includes a circuit for preventing the unbalanced currents and matching the balanced and unbalanced lines with each other.
Some surface acoustic wave devices having the above-described balance-unbalance conversion functions have been the subject of recent patent applications. As a surface acoustic wave device which has a balance-unbalance conversion function and of which the input and output impedances are substantially equal to each other, a device having the configuration shown in FIG. 20 is widely known.
In the surface acoustic wave device shown in FIG. 20, a comb-like electrode (also called an interdigital electrode, defining an Inter-Digital Transducer which is hereinafter referred to as xe2x80x9cIDTxe2x80x9d) 101 is provided on a piezoelectric substrate 100, and IDTs 102 and 103 arranged on the right and left sides (in the surface acoustic wave propagation direction) of the IDT 101.
Moreover, in the aforementioned surface acoustic wave device, reflectors 104 and 105 for reflecting a surface acoustic wave to enhance the conversion efficiency are arranged so as to sandwich the IDTs 101, 102, and 103 from the right and left sides thereof. Moreover, balanced-signal terminals 106 and 107, and an unbalanced signal terminal 108 are provided.
The above-described surface acoustic wave device is called a longitudinally-coupled resonator type surface acoustic wave device of a 3-IDT type. The device has a balance-unbalance conversion function by which a surface acoustic wave is converted between the IDTs 101, 102, and 103.
The surface acoustic wave device having a balance-unbalance conversion function is required to have the following transmission characteristics in the pass-band between the unbalanced-signal terminal and the respective balanced-signal terminals: the amplitude characteristics are the same and the phases are inverted with respect to each other by 180xc2x0. These characteristics are represented by the amplitude-balance-degree and the phase-balance-degree between the balanced-signal terminals, respectively.
The above-described surface acoustic wave device having a balance-unbalance conversion function is assumed to be a three-port device. The unbalanced-input terminal is a first port, and the balanced-output terminals are the second and third ports. The amplitude-balance-degree and the phase-balance-degree are defined as follows: Amplitude-balance-degree=[A]. A=[20 log(S21)]xe2x88x92[20 log(S31)], phase-balance-degree=[Bxe2x88x92180], B=[ZS21xe2x88x92ZS31]. S21 represents the coefficient of transfer from a first port to the second port, and S31 represents the coefficient of transfer from the first port to the third port. The symbol of [ ] in the aforementioned formulae represents an absolute value.
Referring to the balance degrees between signals at the aforementioned balanced-signal terminals, ideally, the amplitude-balance-degree in the transmission band of the surface acoustic wave device is 0 dB, and the phase-balance-degree is 0 degrees.
However, in the configuration of the related art shown in FIG. 20, a problem arises in that the balance degrees between the balanced-signal terminals are deteriorated. Several reasons for this deterioration are speculated. The inventor of the present invention discovered that a capacitance generated between the balanced-signal terminal 106 and the signal line 109, which is a bridging capacitance and is present in the vicinity of the balanced-signal terminal 106, is one of the greatest causes for the deterioration.
Hereinafter, the deterioration of the balance degrees between the balanced-signal terminals, caused by the generation of the bridging capacitance, will be described with reference to FIGS. 21 and 22. The respective characteristics of transmission from the unbalanced-signal terminal 108 to the respective balanced-signal terminals 106 and 107 were excluded from the matching impedance and identified by simulation, and the positions of the resonant modes were examined. FIGS. 21 and 22 show the results.
No bridging capacitance is assumed to be added between the balanced-signal terminal 106 and the signal line 109 in FIG. 20. Then, FIG. 21 shows the difference between the resonant mode of a signal transmitted from the unbalanced-signal terminal 108 to the balanced-signal terminal 106 and that of the signal transmitted from the unbalanced-signal terminal 108 to the balanced-signal terminal 107. FIG. 22 is a graph showing the difference between the resonant modes occurring when a bridging capacitance of 0.30 pF is assumed to be added between the balanced-signal terminal 106 and the signal line 109. In FIGS. 21 and 22, the solid lines represent the characteristics of transmission from the unbalanced-signal terminal 108 to the balanced-signal terminal 106, and the dotted lines represent the characteristics of transmission from the unbalanced-signal terminal 108 to the balanced-signal terminal 107.
Comparison of FIGS. 21 and 22 shows that when the bridging capacitance is added between the balanced-signal terminal 106 and the signal line 109, the difference between the resonant modes of signals at the balanced-signal terminals 106 and 107 which are two balanced-output terminals is larger.
It is generally known that in a longitudinally-coupled resonator type surface acoustic wave device, resonant modes having a phase difference of 180xc2x0 are acoustically coupled to produce the characteristic of the device. In this case, the phases of the resonant modes seen from the balanced-signal terminal 106 side and those seen from the balanced-signal terminal 107 side are different by 180xc2x0 in any case. This is simply expressed by symbols as follows. For example, when three resonant modes seen from the balanced-signal terminal 106 side are expressed by the symbols xe2x88x92, +, xe2x88x92, the three modes seen from the balanced-signal terminal 107 side are expressed by the symbols +, xe2x88x92, +.
When a bridging capacitance having a certain phase angle is added, the resonant mode of a transmission characteristic of a signal transmitted via the balanced-signal terminal 106 and that of the signal transmitted via the balanced-signal terminal 107 are influenced in completely opposite directions by the bridging capacitance, and the balance degrees between the balanced-signal terminals 106 and 107 are deteriorated.
The inventor of the present invention made various examinations including simulation. As a result, the inventor identified the cause for the deterioration of the balance degrees between signals the balanced-signal terminals 106 and 107.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device in which a bridging capacitance, which is a main cause for deterioration of the balance degrees between the balanced-signal terminals of the surface acoustic wave device, is minimized such that the balance degrees between the balanced-signal terminals are greatly improved, and the balance degrees between the balanced-signal terminals are excellent, and the attenuation in the frequency range excluding the transmission band is high.
According to a first preferred embodiment of the present invention, a surface acoustic wave device includes a plurality of interdigital electrodes for converting an input signal to a surface acoustic wave which is to be output, the interdigital electrodes being arranged on a piezoelectric substrate in the surface acoustic wave propagation direction, a plurality of signal terminals including terminals for balanced-signals provided on the piezoelectric substrate, signal lines for connecting the signal terminals and the interdigital electrodes to each other, provided on the piezoelectric substrate, and a ground line provided between a signal terminal and an adjacent signal line on the piezoelectric substrate.
Preferably, in the surface acoustic wave device, the ground line is arranged to surround a signal line that has a pad shape.
In the above-described configuration, a signal input to an IDT (or IDTs) via the signal lines generates a surface acoustic wave on the surface of the piezoelectric substrate, which is propagated and detected by the other IDTs to be output. Thus, the conversion function for the signal is carried out. Moreover, in the above-described configuration, a desired transmission band for a signal can be achieved by adjustment of the intervals between the electrode fingers of the respective IDTs and the number of the electrode fingers. Therefore, the filter function can be also carried out.
In the above-described configuration, a ground line is provided between a signal terminal and the adjacent signal line on the piezoelectric substrate. A bridging capacitance generated between the signal terminal and the signal line is minimized by the ground line. Thus, influences of the bridging capacitance on the signal terminals, especially on the balanced-signal terminals are minimized. That is, the signal propagation characteristic of the device is greatly improved.
Moreover, according to another preferred embodiment of the present invention, a surface acoustic wave device includes a plurality of interdigital electrodes for converting an input signal to a surface acoustic wave which is to be output, the interdigital electrodes being arranged on a piezoelectric substrate in the surface acoustic wave propagation direction, a plurality of signal terminals including terminals for balanced signals provided on the piezoelectric substrate, signal lines for connecting the signal terminals and the interdigital electrodes to each other, provided on the piezoelectric substrate, and a ground electrode finger provided in the outermost portion of at least one of two adjacent interdigital electrodes which is opposed to that of the other interdigital electrode in such a manner that the tip of the ground electrode finger extends in the interdigitation-span direction compared to the other electrode fingers of the interdigital electrode.
In the above-described configuration, since the ground electrode finger is provided on the piezoelectric substrate in such a manner that the tip of the ground electrode extends in the interdigitation-span direction, the ground electrode finger can be interposed between the signal terminal and the adjacent signal line.
Accordingly, in the above-described configuration, a bridging capacitance generated between the signal terminal and the adjacent signal line is minimized by the ground line. Thus, influences of the bridging capacitance on the signal terminals, especially on the balanced signal terminals are minimized. That is, the signal propagation characteristic of the device is greatly improved.
Furthermore, according to another preferred embodiment of the present invention, a surface acoustic wave device includes a plurality of interdigital electrodes for converting an input signal to a surface acoustic wave which is to be output, the interdigital electrodes being arranged on a piezoelectric substrate in the surface acoustic wave propagation direction, a plurality of signal terminals including terminals for balanced signals provided on the piezoelectric substrate, and signal lines for connecting the signal terminals and the interdigital electrodes to each other, provided on the piezoelectric substrate, the interdigital electrode connected to a signal line and an adjacent interdigital electrode being constructed such that the widths of the bus bars of the interdigital electrodes are reduced whereby the opposing portions of the interdigital electrodes are reduced in size as compared to the other interdigital electrodes.
In the above-described configuration, the interdigital electrode connected to a signal line and the adjacent interdigital electrode are constructed such that the widths of the bus bars of the interdigital electrodes are reduced whereby the opposing portions of the interdigital electrodes are reduced in size as compared to the other interdigital electrodes. Thus, a bridging capacitance between the IDTs is minimized, and influences of the bridging capacitance on the signal terminals, especially on the balanced-signal terminals are minimized. Thus, the signal propagation characteristic of the device is greatly improved.
In the surface acoustic wave device according to various preferred embodiments of the present invention, preferably, a surface acoustic wave resonator connected to an interdigital electrode may be provided on the piezoelectric substrate. In this configuration, the surface acoustic wave resonator is connected in series with the IDTs so as to overlap their anti-resonance points, whereby the attenuation characteristic of the device is greatly improved. Moreover, the surface acoustic wave resonator may be connected, e.g., in parallel to the IDTs so as to make the resonance point coincident with the attenuation point, whereby a steep attenuation incline can be obtained.
The above-described surface acoustic wave device may have a balance-unbalance conversion function. In this configuration, the difference between the resonant modes of signals at the balanced-signal terminals is minimized. Thus, the balance-unbalance conversion characteristic of the device is greatly improved, that is, the balance degrees between the balanced-signal terminals are highly satisfactory, and the attenuation occurring out of the transmission band becomes very large.
In the surface acoustic wave device, preferably, the number of electrode fingers of the IDT connected to a balanced-signal terminal is even. In this configuration, since the number of electrode fingers of the IDT connected to a balanced-signal terminal is even, the balance degrees between the balanced-signal terminals are greatly improved.
Other features, characteristics, elements, and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.